A technique for forming a transistor by using a semiconductor thin film formed over a substrate having an insulating surface has attracted attention. Such a transistor is applied to a wide range of semiconductor electronic devices such as an integrated circuit (IC) and an image display device (also simply referred to as display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to the transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a technique is disclosed by which a transistor is manufactured using zinc oxide or an In—Ga—Zn-based oxide as an oxide semiconductor (see Patent Documents 1 and 2).